1. Field of the Invention
This invention relates to controlling humidity in a closed environment and more particularly relates to the use of versatile absorbers for controlling humidity in a product environment such as within the enclosure of a hard disk drive (HDD).
2. Description of the Related Art
Moisture sensitive products, including electronic products such as hard disk drives (HDD), typically require the use of some sort of absorbent material packaged within the products in order to prevent mechanical or electrical failure caused by condensation or contamination. In some cases, humidity and contamination can cause a loss of data, either by damaging the disk, damaging the head, or preventing the correct reading of the data on the disk by the head. The absorbent material is typically some type of desiccant such as silica gel, calcium sulfate, or montmorillonite clay. In the case of HDDs, it is imperative that the absorber material itself not interfere with the functionality of the HDD. Therefore, the absorber material is conventionally contained within a plastic outer package with a permeable membrane cover which is then secured within the HDD enclosure.
Because the physical size of HDDs is constantly decreasing due to the demand for smaller devices such as laptops and mp3 players, it has also become important that the absorbent material part not take up a large amount of space within the physical device. Typically, this is accomplished by simply scaling down known absorber material packages to fit within the shrinking space of an HDD enclosure. However, this approach also inherently leads to smaller amounts of absorber material fitting within the confines of the smaller packages. In some cases, the smaller amount of absorber material may be insufficient to reasonably manage the gases and humidity within the closed environment of the HDD.
For example, a laptop may be exposed to very high temperatures if left in a car during the summer, and conversely, the laptop may immediately be exposed to very low temperatures if the laptop is brought into an air-conditioned building. This may lead to the formation of condensation on the inside of the HDD as a result of the sudden temperature change. Thus, an absorber material placed inside the HDD regulates the amount of moisture and other gases within the HDD enclosure so as to prevent them from adversely affecting the operation of the HDD.
Because even very small particles of dust can inhibit the proper operation of a HDD, absorbent material packages must be clean. In other words, the packages must be constructed from materials which can be cleaned prior to their introduction into the HDD so as to avoid contamination within the enclosure. Conventional absorbent material packages typically utilize a material called polytetrafluoroethylene (ePTFE) for such clean applications.
In one conventional embodiment, the absorber material may be entirely enclosed within a package made from ePTFE laminate. In order to prevent tearing of the ePTFE laminate, a thickness of approximately 0.075 inches or more may be required. Furthermore, ePTFE laminate has limited elasticity. Thus, when a tablet form of the absorber material is used, the ePTFE laminate package may be susceptible to tearing or may be unable to compressively hold the tablet in position within an enclosure. ePTFE also has a very low coefficient of friction such that a part made with walls of ePTFE is not suitable to frictionally secure itself into a hole or enclosure. Additionally, the process of making packages entirely from ePTFE appears to be unsuitable for large scale or high speed operations.
In an alternative conventional embodiment, a package for holding the absorber material may be formed by using injection molding techniques. In this embodiment, a rigid base is formed by injecting molten plastic into a mold. Upon cooling, an absorber material, such as a desiccant tablet, is placed in the base, and an ePTFE cover is sealed to the base to complete the enclosure. Again, because the base is rigid and has little flexibility, it is typically unable to compressively hold itself in position and may require the use of a screw, spring, or some type of adhesive in order to fasten it into position. Although injection molding techniques can be carried out in multicavity hot runner molds, the tools are very expensive and must be replaced periodically.
One of the limitations of currently available methods for packaging an absorber material is the minimum thickness of the material being used. As the parts decrease in size, the space occupied by the walls of the part may occupy a large percentage of the total space allotted for the part, thereby leaving less space available for the absorber material.
For example, assume that the external dimensions of the part, in order to fit into a hard drive enclosure, must not exceed 10.0 mm (0.39 inches) in length, 10.0 mm (0.39 inches) in width, and 10.0 mm (0.39 inches) in height for a total volume of 1000 mm3. Then the total interior volume of the part, or the amount of space available for occupation by the absorber material, becomes a function of the thickness of the material used to form the walls of the part. Thus, if the package is constructed from material that is approximately 2.0 mm (0.075 inches) thick, then the usable interior volume of the device (assuming a six wall enclosure made from materials of the same thickness) is approximately 216 mm3. (10−2−2=6 and 6×6×6=216). This means that approximately 784 mm3 is occupied by the wall forming material. Therefore, even with this simplified example, it is apparent that a reduction in the thickness of the walls may result in a dramatic increase of space available for the storage of the absorber material.
For instance, assume now that a part with the same restrictions listed above is made using material with a thickness of about 0.25 mm (0.010 inches). The interior volume now available inside the part for storage of the absorber material increases to approximately 857 mm3. (9.5×9.5×9.5=857.375), while the amount of space occupied by the walls of the part decreases to approximately 143 mm3. Thus, by reducing the wall thickness of materials used for holding an absorber material, it is possible to significantly increase the amount of absorber material contained inside the part without increasing the exterior size of the part. An additional drawback of conventional parts is that they may absorb vapors too rapidly for a particular application. For example, magnetic storage devices commonly employ a desiccant to protect against a rise in moisture during storage or operation, or to buffer the HDD enclosure at a specific humidity range during changes in temperature. However, typical absorber parts absorb vapors very rapidly such that they absorb significant amounts of vapor during time it takes to install the part into a sealed or semi-sealed environment.
Conventionally, the absorber parts are stored in protected containers or purged with dry gas prior to installation which takes place in a clean room environment. In some cases, attempts made to purge the part with dry gases are performed immediately after installation and just prior to sealing the closed environment. However, this method is often ineffective at sufficiently drying out the absorber material and can be costly. Therefore, by providing a way to restrictively control the rate of absorption, a part may be engineered such that it can be quickly installed without absorbing a substantial amount of ambient vapors while still providing the absorption rates necessary to perform in a closed environment.